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Background Statement for SEMI Draft Document 5959A
REVISION OF SEMI PV25-1011
TEST METHOD FOR SIMULTANEOUSLYMEASURING OXYGEN, CARBON, BORON AND PHOSPHORUS IN SOLAR SILICON WAFERS AND FEEDSTOCK BY SECONDARY ION MASS SPECTROMETRY
Notice: This background statement is not part of the balloted item. It is provided solely to assist the recipient in reaching an informed decision based on the rationale of the activity that preceded the creation of this Document.
Notice: Recipients of this Document are invited to submit, with their comments, notification of any relevant patented technology or copyrighted items of which they are aware and to provide supporting documentation. In this context, “patented technology” is defined as technology for which a patent has issued or has been applied for. In the latter case, only publicly available information on the contents of the patent application is to be provided.
Background
Doc. 5959 was sent out for reapproval at the Spring 2016 meeting. Negatives were received. The task force incorporates those negatives and issues ballot 5959A.
Notice: Additions are indicated byunderlineand deletions are indicated by strikethrough.
Review and Adjudication Information
Task Force Review / Committee AdjudicationGroup: / International PV Analytical Test Methods, Metrology, and Inspection TF / PV Materials NA TC Chapter
Date: / Nov 9, 2016 / Nov 9, 2016
Time & Timezone: / 9:00-11:00 AM PDT / 11:00 AM -12:00 PM PDT
Location: / SEMI HQ / SEMI HQ
City, State/Country: / San Jose, CA/USA / San Jose, CA/USA
Leader(s)/Authors: / Hugh Gotts (Air Liquide) / Hugh Gotts (Air Liquide)
Standards Staff: / Kevin Nguyen ( ) / Kevin Nguyen ( )
This meeting’s details are subject to change, and additional review sessions may be scheduled if necessary. Contact the task force leaders or Standards staff for confirmation.
Telephone and web information will be distributed to interested parties as the meeting date approaches. If you will not be able to attend these meetings in person but would like to participate by telephone/web, please contact Standards staff.
Check on calendar of event for the latest meeting schedule.
SEMI DRAFT DOCUMENT 5959A
REVISION OF SEMI PV25-1011TEST METHOD FOR SIMULTANEOUSLYMEASURING OXYGEN, CARBON, BORON AND PHOSPHORUS IN SOLAR SILICON WAFERS AND FEEDSTOCK BY SECONDARY ION MASS SPECTROMETRY
1 Purpose
1.1 Secondary ion mass spectrometry (SIMS) can measure the total bulk concentrations of oxygen, carbon, boron and phosphorus in polished solar silicon wafers and silicon feedstock. Bulk carbon is important because it can form carbon-related defects, such as SiC inclusions. Bulk oxygen is important in boron-doped silicon because a BOx defect can degrade cell efficiency. Boron and phosphorus are common dopants in solar Si wafers, and are difficult to measure directly in solar Si wafers, especially in highly compensated silicon.
1.2 The purpose of this test method is the measurement of all four elemental concentrations can be accomplished in one test and using one SIMS instrument equipped with a cesium primary ion source.
1.3 The SIMS method can be used at three stages of solar Si wafers and silicon feedstock: research and development; process check; and verification at the commercial sales/purchase interface.
2 Scope
2.1 This Test Method covers the simultaneous determination of total oxygen, carbon, boron and phosphorus concentrations in the bulk of silicon samples that are prepared from solar silicon wafers or solar silicon feedstock using SIMS.[1]
2.2 This test method can be used at three stages of solar Si wafers and silicon feedstock: research and development; process check; and verification at the commercial sales/purchase interface.
2.2 2.3 This Test Method can be used for silicon in which the dopant concentrations by this method are determined to be less than 0.2% (1 × 1020 atoms/cm3) for oxygen, carbon, oxygen, boron, and phosphorus and 5 × 1016 atoms/cm3; 1 × 1016 atoms/cm3; 1 × 1014 atoms/cm3; and 2 × 1014 atoms/cm3 , respectively.
2.3 2.4 This Test Method can be used for silicon irrespective of the amount of compensation in the silicon within the limits of elements specified in 2.3.
2.4 2.5 This Test Method can be used for either multi-crystalline or single crystal silicon wafers.
2.5 2.6 This Test Method is for bulk analysis where the oxygen, carbon, boron and phosphorus concentrations are constant with depth.
2.6 This Test Method can be used for silicon in which the oxygen, carbon, boron and phosphorus contents are respectively 5 × 1016 atoms/cm3; 1 × 1016 atoms/cm3; 1 × 1014 atoms/cm3; and 2 × 1014 atoms/cm3.
2.7 Calibration of the SIMS measurement of boron and phosphorus in silicon is traceable to NIST reference materials. Calibration for the oxygen measurement is indirectly traceable to NIST reference material through FTIR correlation. Calibration for the carbon measurement is completed using carbon-implanted silicon wafer reference materials.
NOTICE: SEMI Standards and Safety Guidelines do not purport to address all safety issues associated with their use. It is the responsibility of the users of the documents to establish appropriate safety and health practices, and determine the applicability of regulatory or other limitations prior to use.
3 Limitations
3.1 Oxygen and carbon on or in the surface silicon oxide can interfere with the bulk oxygen and carbon measurements. This can beis minimized by baking the samples before analysis.
3.2 Oxygen and carbon adsorbed on the test specimen surface from the SIMS instrument chamber and fixtures interfere with the bulk oxygen and carbon measurements by raising the background signals. The vacuum quality of the SIMS instrument can be used to minimize this.
3.3 Oxygen or carbon in the SIMS primary Cs beam may be implanted into the silicon specimen as CsO or CsC and thereby increase the oxygen and carbon background concentrations. A primary beam mass filter may be used to reduce this interference, but in this case, the reduced Cs beam current density will reduce the sputter rate which may increase the oxygen and carbon background signals.
3.4 1H30Si at mass 31 can interfere for detecting phosphorus 31P. This ismust be avoided by using SIMS equipment with a minimum mass resolution of 4000.
3.5 The specimen surface must be flat in the specimen holder window so that the inclination of the specimen surface with respect to the ion collection optics is constant from specimen to specimen. Otherwise, the accuracy and precision can be degraded. This is accomplished by mechanical polishing the silicon specimen.
3.6 Calibration of the carbon measurement in silicon by SIMS is not traceable to a NIST reference material. Instead carbon implantation into silicon is used for reference. Bias in the assigned carbon areal density of the calibration specimen can introduce bias into the SIMS carbon measurement.
3.7 Variability from the calibration measurement of any of the four elements may increase the measurement precision of the test specimen.
3.8 Bias in the assigned carbon areal density of the calibration specimen can introduce bias into the SIMS measured carbon.
3.9 3.8 The detection capability depends upon the SIMS instrumental backgrounds and the precision of the measurements.
3.10 3.9 The analytical volume of this test method is on the order of 100’s of micrograms, so that heterogeneity of oxygen, carbon, boron, or phosphorus concentrations on scales greater than this analytical volume may degrade the inherent precision of the repeatability of thisTest Method comparable to the variation in lateral homogeneity.
4 Referenced Standards and Documents
4.1 SEMI Standards and Safety Guidelines
SEMI MF2139 — Test Method for Measuring Nitrogen Concentration in Silicon Substrates by Secondary Ion Mass Spectrometry
4.2 ASTMStandards[2]
ASTM E122 — Practice for Calculating Sample Size to Estimate, With a Specified Tolerable Error, the Average for a Characteristic of a Lot or Process
ASTM E673 — Terminology Relating to Surface Analysis
NOTICE: Unless otherwise indicated, all documents cited shall be the latest published versions.
5 Terminology
5.1 Definitions— all terms in this test method are in accordance with those given in ASTME673.
6 Summary of Test Method
NOTE 1:This Test Method is almost identicalsimilar to SEMIMF2139 except for the elements to be measured.
6.1 SIMS is utilized to determine the bulk concentration of oxygen, carbon, boron and phosphorus in multicrystalline or single crystal silicon. Specimens of silicon are mechanically polished to provide a flat analysis surface. The polished silicon specimen (one or more calibration specimen and the test specimens) are loaded into a sample holder. The holder with the specimens is baked at 100°C in air for 1 hour and then transferred into the analysis chamber of the SIMS instrument.
6.2 The calibration samples are analyzed using a cesium (Cs) primary ion beam and negative ion spectrometry of 16O, 12C, 31P, and 11B28Si to determine a relative sensitivity factor (RSF) of oxygen, carbon, phosphorus and boron in silicon.
6.3 Each test specimen in the sample holder is sputtered by the primary cesium ion beam, without analyzing secondary ion intensities, in order to reduce the oxygen and carbon backgrounds. The amount of time required for this pre-analysis sputtering depends upon the instrumentation and the desired background oxygen and carbon concentrations.
6.4 For the analysis of the test specimens, a cesium primary ion beam is used to bombard each test specimen at two different sputter rates, by reducing the beam raster area for the second sputter rate, while at the same time maintaining a constant detected area. This beam raster change technique is used for two reasons. Firstly, this technique reduces the background oxygen and carbon contributions from the sample surface. Secondly, this technique allows the determination of both the instrumental oxygen and carbon backgrounds and the real bulk oxygen and carbon concentrations in each test specimen measurement without the use of a separate blank specimen, and due to averaging is capable of separating the bulk oxygen and carbon from the instrumental oxygen and carbon even if the bulk oxygen and carbon are less than the instrumental oxygen and carbon.[3]
6.5 The two sputter rates, and the amount of time they are applied in each measurement, are chosen to optimize the detection capability; the values depend on the instrumentation being used. However, the second sputter rate is typically the maximum sputter rate of the instrumentation, and the first sputter rate is typically less than half that of the second sputter rate.
6.6 The negative secondary ions of 12C, 16O, 31P, and 11B28Si are mass analyzed by a mass spectrometer, and detected by an electron multiplier (EM) or equivalent high-sensitivity ion detector as a function of time. The matrix negative secondary ion count for silicon such as 28Si is measured by a Faraday cup (FC) or appropriate detector during the profile. If multiple detectors are used during the test, the relative sensitivities of the detectors are determined by measuring standard ion signals (either the same negative secondary ion count rate or ion count rates of known relative intensity such as natural 28Si/30Si) on each detector.
6.7 For illustration purposes an example data profile is shown in Figure1 where the log of counts of 12C, 16O, 31P, and 11B28Si and 28Si is plotted versus cycle time. One cycle is the time it takes the SIMS detection scheme to cycle the mass spectrometer in order to mass analyze all four elements; this is typically a few seconds per cycle for a magnetic sector spectrometer. At about 12 cycles in the plot, the Cs beam raster area was changed from 250 m × 250 m down to 100 m × 100 m, resulting in an increase in signal intensities, because the volume of material analyzed per second has increased. At about 24 cycles, the Cs beam raster area was reversed back to the first condition, and the signal intensities are seen to be reduced back to the original levels. Since the surface analysis area was kept constant during the raster change (i.e., the actual surface area detected is independent of the raster area), the signal intensities have two separate contributions, one from elements that are adsorbed to the surface and one from elements in the volume of material analyzed each second. There is some oxygen and carbon that is adsorbed to the surface from the local environment outside of the silicon, and there is some oxygen and carbon that is from the bulk silicon. A key point is that if there is no oxygen or carbon in the bulk silicon, the oxygen or carbon signal intensity does not change as a result of the change in sputter rate, because the adsorbed oxygen or carbon (i.e., instrumental oxygen or carbon signal) is only dependent on the analysis area, which has been held constant. This understanding leads to the following equations for each of the four elements:
(1)
(2)
(3)
(4)
where:
N and n refer to the identify of any one of the elements being measured.
In = secondary ion intensity of the ion under the higher sputter rate conditions,
IN = secondary ion intensity of the ion under the lower sputter rate conditions,
[Nb] = contribution of background element to the observed element concentration under the higher sputter rate conditions,
[NB] = contribution of background element to the observed element concentration under the lower sputter rate conditions,
IB = secondary ion intensity from the adsorbed background element under both sputter rate conditions,
Isi = secondary ion intensity of the silicon matrix (e.g., 28Si) under the higher sputter rate conditions,
ISI = secondary ion intensity of the silicon matrix (e.g., 28Si) under the lower sputter rate conditions,
RSF = relative sensitivity factor for converting the ion intensity ratio to concentration, and
[N] = bulk element concentration in the test specimen.
6.8 Equations (1) through (4) can be used to determine [N] and [Nb] from measured data as follows:
(5)
(6)
In these equations, the secondary ion intensities used are averages of these signal intensities. In the example shown in Figure1, the lower sputter rate was used at the beginning of the profile, and the higher sputter rate in the middle of the profile. For this example, the bulk carbon concentration was determined to be 2.5 × 1017/cm3 and the instrumental carbon background concentration was 9× 1015/cm3. The use of averages for In and IN is particularly critical for detecting low levels of the element where the signal intensity count rates can be low and therefore noisy. By using averages, it is possible to detect an [N] level that is less than [Nb] in the analysis.
6.9 The SIMS measurement of oxygen in bulk-doped silicon sometimes reveals anomalous intensity spikes that are greater than a random fluctuation of signal intensity, even in silicon that has not been thermally processed beyond crystal growth. These oxygen intensity spikes are not well understood, but are suspected to be associated with oxygen defects, possibly an aggregation of oxygen in a precipitate form. SIMS effects have been well known for oxygen in thermally-processed silicon.[4] The presence of such nonrandom signals, if significant, can make the averaging method used here questionable. This effect does not occur for carbon, boron or phosphorus aggregation.
NOTE:Top curve Mass 28 (28Si); middle curves Mass 12 (12C) and Mass 16 (16O),
bottom curves Mass 31 (31P) and Mass 39 (11B28Si)
Figure 1
SIMS Signals in Highly Compensated Solar Silicon, Log Scale
7 Apparatus
7.1 SIMS Instrument— Equipped with a cesium primary ion source, electron multiplier detector, and Faraday cup detector, capable of measuring negative secondary ions with a mass resolution of at least 4000. The SIMS instrument should be adequately prepared (i.e., baked) so as to provide the lowest possible instrumental background, since the instrumental background can affect the detection capability. High quality vacuum is desired. A liquid nitrogen- or liquid helium-cooled cryopanel, which surrounds the test specimen holder in the analysis chamber, may be helpful.
7.2 Test specimen holder.
7.3 Sample polishing equipment.
7.4 Diamond saw for cutting the silicon specimen from a solar silicon wafer or silicon feedstock.
7.5 Oven— For baking the test specimen holder.
7.6 Stylus Profilometer — Or equivalent device to measure SIMS crater depths. This device is required to calibrate depth scale for concentration profiles of calibration standard samples.
8 Reagents and Materials
8.1 For Polishing — Diamond lapping film and silica solution are required for this Test Method.
9 Safety Precautions
9.1 The preparation of silicon test specimens from silicon substrates requires the crystal to be cleaved or broken. This procedure may generate very sharp silicon shards. Care must be taken to protect the body and particularly the eyes from these shards.
10 Sampling, Test Specimens, and Test Units
10.1 Since this procedure is destructive in nature, a sampling procedure must be used to evaluate the characteristics of a group of silicon wafers. No general sampling procedure is included as part of this Test Method, because the most suitable sampling plan depends upon individual conditions. For referee purposes, a sampling plan shall be agreed upon before conducting the test. See ASTME122 for suggested choices of sampling plans.
10.2 The calibration or reference material shall be Floating Zone silicon crystal wafer(s) with oxygen, carbon, boron, or phosphorus implanted at doses of about 1 × 1014 atoms/cm2 and energies of about 100keV in silicon where the bulk concentration for the implanted element is less than 1 × 1017 atoms/cm3.
10.3 For traceability of oxygen, boron and phosphorus use Certified Reference Materials.to NIST: Certified Reference Materials SRM2551, SRM2137, and SRM2133. Alternative to the NIST Certified Materials would be secondary reference materials that are traceable to the NIST Certified Materials.